Aluminum bronze alloy having improved resistance to intergranular oxidation by the addition of tin and silver



weight of the alloy of limited States Patent Oflice AL MINUM BRONZE ALLOY HAVING IMPROVED RESISTANCE TO INTERGRAN'ULAR OXIDA- TION BY THE ADDITION OF TIN AND SILVER John F. Klement, Milwaukee, Wis., assiguor to Ampoo Metal, Inc, Milwaukee, Wis., a corporation of Wisconsm N Drawing. Application July 5, 1956 Serial No. 595,831 Claims. (Cl. 75-154) This invention relates to an aluminum bronze alloy having increased resistance to intergranular oxidation when subjected to stress in a hot water or steam atmosphere.

It is Well known that almost all copper-base alloys, when stressed in certain corrosive media, are subject to a particular species of oxidation,

ing of a metal subjected to contact with a corrodent and This invention involves granular oxidation when stressed in media containing hot Water and steam.

It has been found that the oxidation or fracture of an a urninum bronze alloy under the action of stress and water or steam corrodent, follow paths which are intergranular in nature. It is believed that the reason for this intergranular type of attack is that aluminum atoms are in a higher concentration in the grain boundary than in the grains. This is probably because aluminum atoms have a negative elastic energy interaction with dilated grain boundary regions; therefore the larger aluminum atoms become segregated in the grain boundary and are subject to oxidation.

The problem of intergranular attack is particularly serious in alpha-phase aluminum bronze alloys which contain about 5% to 8% aluminum. The alpha-phase alloy has excellent working properties and can be readily fabricated into tube, sheet, plate, etc., in which form the alloy is often subjected to stress in a hot water or steam atmosphere.

The present invention is based on the discovery that the resistance of an alpha-phase aluminum bronze alloy to intergranular oxidation when subjected to a steam or hot water atmosphere can be increased by the addition of small amounts of tin and silver to the alloy. This is particularly true where the amount of tin and silver is maintained within a specific ratio to the aluminum content in the alloy.

More specifically, the total tin and silver content in the alloy should be in the ratio of about one part by tin and silver to 20 partsby Weight of aluminum. It has been found that an aluminum bronze alloy containing the above aluminum to tin and silver ratio is much more resistant to intergranular oxidation when stressed in hot water and steam because tin and silver atoms, being larger than aluminum atoms, have a larger negative elastic energy interaction with dilated grain boundary regions than aluminum atoms, and therefore tend to replace aluminum atoms in the grain boundary. This resistance to oxidation is probably true because of the low heats of formation of the oxides of tin and silver and thus the grain boundaries of the aluminum bronze alloy containing tin and silver show unusual resistance to intergranular attack when stressed in steam and hot Water media.

It has been found that it is necessary to make the improved alloy with a certain critical ratio of tin and silver to the aluminum content. This is believed due to the 2,829,969 Patented Apr. 8, 1958 additional theoretical consideration of the configurational entropy contributed by the aluminum, tin and silver In effect, this means that the smaller the concentration of a particular solute atom, the more dilficult alloying with copper in the previous art has not specitially prevent intergranular attack when stressed in hot Water and steam environs. For instance, tin additions have been made to brasses along with nickel, and the tin in this case combines with the nickel to form a compound which through proper heat treatment can be made to substantially harden the brass. Silver has been used in prior art mainly in conjunction with copper to improve the mechanical properties of conductivity alloys and the recrystallization characteristics of copper. In contrast, this invention relates to an alpha-phase aluminum bronze alloy, with greatly improved resistance to intergranular oxidation when stressed in hot water and steam media through the use of a specific aluminum to tin and silver ratio. In this invention, the mechanical properties of the alpha-phase aluminum bronze alloy are maintained, and at the same time the resistance of the alloy to intergranular attack is increased.

According to the invention, the general composition range, by weight, of the alloy is as follows: Aluminum 5.0 to 8.0%. Iron 1.6 to 2.9%.

Tin-silver alloy In weight ratio of 1 Balance.

part alloy to parts of aluminum. Copper The controlled preferred compositional range of the alloy is as follows, in weight percent:

Aluminum 6.00 to 6.75%.

Iron 2.00 to 2.40%

Tin-silver alloy In a Weight ratio of 1 part alloy to 18 to 27 parts of aluminum.

percent provides the following compositional range:

Percent Aluminum 5.08.0 Iron 1.6-2.9 Tin 005-.69 Silver DOS-.69 Copper Balance Converting the preferred compositional range to weight percent provides the following compositional range:

Percent Aluminum 6.00-6.75 Iron 2.00-2.40 Tin .01.36 Silver .01.36 Copper ....e Balance A specific illustration of an alloy falling within the above ranges is as follows, in weight percent:

, Percent Aluminum 6.25 Iron 2.08 Tin 0.16 Silver 0.15 Copper 91.36

In addition to the ratio between the aluminum and tin and silver, it has been found that the most desirable properties are obtained in the alloy when the iron content is maintained at a 1 to 3 ratio to the aluminum content. The iron is employed as a grain stabilizer and also acts to increase the strength of the alloy, particularly at higher temperatures. The 1 to 3 ratio between the iron and aluminum provides the alloy with the most desirable grain refining characteristics and mechanical properties.

In addition, other metals such as nickel and manganese may be employed in the alloy in amounts up to about 0.25% to add to the mechanical properties of the alloy, if desired.

Age hardening metals such as silicon, beryllium and the like should be avoided in the present alloy. Nickel, if used alone in the aluminum-iron copper alloy of the invention, will not age harden, but if a combination of metals such as silicon and nickel is used, the combination will age harden when the alloy is heated during welding, forming, or the like to form nickel silicides. The nickel silicides are extremely brittle and tend to precipitate and gather at the grain boundaries. This tendency of the age hardening precipitate to gather at the grain boundary is believed to prevent the tin and silver from gathering at the grain boundaries and in effect destroys the purpose of the tin and silver addition. The age hardening precipitates can be removed by a solution quench from over 1600 F., but a heat treatment such as this is not possible with a finished or fabricated article because the dimensional tolerances of the article will be destroyed by the'heat treatment. Therefore, age hardening constituents should be avoided in the present alloy.

The alloy is formed by initially preparing a pre-alloy of copper, aluminum and iron. This pre-alloy is then alloyed with additional copper and tin and silver and cast. It is preferred not to add the tin and silver to the pre-alloy because a portion of the tin and silver would be lost by oxidation during melting and additional quantities of tin and silver would have to be employed to hold the concentration of the same within the desired limits.

The present alloy can be readily fabricated into the form of rod, sheet, plate, tube, etc. or can be used as a weld deposit. The alloy can be made to provide a yield strength from 20,000 p. s. i. to 90,000 p. s. i., a tensile strength of 40,000 p. s. i. to 120,000 p. s. i. and an elongation in 2 inches of 1% to 60%, depending on the alloy composition, the method of fabrication employed and the heat treatment used. For example, an alloy having 6.25% aluminum, 2.10% iron, 0.22% tin, 0.08% silver and the balance copper, in the form of inch thick plate, can be hot rolled and annealed to give a tensile strength of 80,000 13. s. i., a yield strength of 42,000 p. s. i., an elongation in 2 inches of 38% and a Brinell hardness of 150.

The same alloy in the form of cold drawn 4 diameter rod has a tensile strength of 85,000 p. s. i., a yield strength of 55,000 p. s. i., an elongation in 2 inches of 30% and a Brinell hardness of 165.

As an example of the alloy as a weld deposit, the composition consisted of 7.0% aluminum, 2.4% iron, 0.25% tin and the balance copper. A sample of this alloy, taken through a butt welded joint, had a tensile strength of 75,000 p. s. i., a yield strength of 35,000 p. s. i., an elongation in 2 inches of 25% and a Brinell hardness of 150.

The alloy has high resistance to intergranular fracture resulting from oxidation at the grain boundaries. For example, the alloy will resist intergranular fracture for at least 2000 hours when stressed from 20,000 to 50,000 p. s. i. in a steam or hot water atmosphere at temperatures from 180 F. to 500 F. In some cases the alloy has been stressed for 8000 hours under the above conditions without failure.

As an example of the increased resistance of the alloy to intergranular oxidation when stressed in a hot water or steam atmosphere, a specimen of an alloy containing 6.00% aluminum, 2.20% iron, 0.22% tin, 0.08% silver and the balance copper was stressed beyond the yield strength by deflection bending in a media consisting of a mixture of a 5% solution of acetic acid and an 8% solution of sulphuric acid, at a temperature of 350 F. The specimen did not fail in 1000 hours of testing.

A second specimen having a similar composition, but without tin and silver additions, was stressed under identical conditions and failed after hours of testing.

The present invention provides an aluminum bronze alloy that has greatly improved resistance to intergranular oxidation when stressed in steam and water atmospheres. The term steam is intended to include gaseous mixtures of steam and other vapors and the term hot water is intended to include heated aqueous solutions of salts, acids or bases and other materials containing water or moisture as an ingredient.

Various modes of carrying out the invention are contemplated as being within the scope of the following claims particularly pointing out and distinctly claiming the subject matter which is regarded as the invention.

I claim:

1. An aluminum bronze alloy consisting essentially by weight of: about 5.0% to 8.0% aluminum; about 1.6% to 2.9% iron; about 0.005% to 0.69% tin; about 0.005 to 0.69% silver; and the balance substantially copper, said alloy characterized by withstanding intergranular fracture for at least 2000 hours when stressed from 20,000 to 50,000 p. s. i. in a steam or hot water atmosphere at a temperature of F. to 500 F.

2. An aluminum bronze alloy having increased resistance to intergranular oxidation when subjected to stress in a hot water or steam atmosphere, consisting essentially by weight of: about 6.00% to 6.75% aluminum; about 2.00% to 2.40% iron; about 0.01% to 0.36% tin; about 0.01% to 0.36% silver; and the balance substantially copper.

3. An aluminum bronze alloy having increased resistance to intergranular oxidation when subjected to stress in a hot water or steam atmosphere, consisting essentially of by weight: about 5 .0% to 8.0% aluminum; iron in a weight ratio of about 1 part iron to about 3 parts aluminum; about 0.005% to 0.69% tin; about 0.005% to 0.69% silver; and the balance substantially copper, said tin and silver segregating in the grain boundaries of the alloy and restricting the oxidation of said alloy at said grain boundaries.

4. An aluminum bronze alloy having increased resistance to intergranular oxidation when subjected to stress in a medium containing hot water or steam, consisting essentially of by weight: about 6.00% to 6.75% aluminum; iron in a weight ratio of about 1 part iron to about 3 parts aluminum; about 0.01% to 0.36% tin; about 0.01% to 0.36% silver; and the balance substantially copper.

5. An aluminum bronze alloy, consisting essentially of by weight: 6.25% aluminum; 2.08% iron; 0.16% tin; 0.15% silver; and the balance copper, said alloy having a high concentration of tin in the grain boundaries to thereby increase the resistance of the alloy to intergranular oxidation when subjected to stress in a medium containing hot water or steam.

No references cited. 

1. AN ALUMINUM BRONZE ALLOY CONSISTING ESSENTIALLY BY WEIGHT OF: ABOUT 5.0% TO 8.0% ALUMINUM; ABOUT 1.6% TO 2.9% IRON; ABOUT 0.005% TO 0.69% TIN; ABOUT 0.005 TO 0.69% SILVER; AND THE BALANCE SUBSTANTIALLY COPPER, SAID ALLOY CHARACTERIZED BY WITHSTANDING INTERGRANULAR FRACTURE FOR AT LEAST 2000 HOURS WHEN STRESSED FROM 20,000 TO 50,000 P.S.I. IN A STEAM OR HOT WATER ATMOSPHERE AT A TEMPERATURE OF 180*F. TO 500*F. 